In the field of semiconductor device manufacturing, an atomic layer deposition (hereinafter referred to as ALD) attracts attention as a technique to form a certain film on a semiconductor wafer (hereinafter simply referred to as a wafer) which is a substrate.
According to the ALD technique, a film having a desired thickness is formed by repeating plural times an operation of forming a thin unit film, which is almost monomolecular layer. The operation of forming a thin unit film includes: sequentially supplying processing gases (precursors) for forming a desired film into a processing space, where a target substrate is placed, in a processing chamber; purging (substituting) excessive processing gases with a purge gas (substitution gas) after the supplying of the processing gases; and reacting the processing gases with each other on the wafer to form a thin unit film (see, e.g., Japanese Patent Application Publication No. 2004-091874). As an example, a method to form a TiN film is known, in which TiCl4 gas and NH3 gas are used as the processing gases, and supplying each of the gases is repeated several times, with purging being carried out between the supplying of each of the gases.
The ALD technique is advantageous in that film thickness can be controlled with a high precision by the number of repeated cycles, and in that a film of good quality can be produced at a low temperature with excellent step coverage.
In the ALD technique, purging of the processing gases is carried out after the supplying of each of the processing gases in order to remove excessive processing gases from the processing space. Consequently, reactions occur only on a surface of a wafer, thereby maintaining good control of film thickness. Additionally, the purging is carried out to prevent reaction products, which contribute to particles, from being generated when the processing gases react with each other in the processing space.
In addition, in order to improve the control over film thickness in the ALD technique, it is necessary to increase purge efficiency so as to improve substitution rate of gases in the processing space during the purging. To this end, it is preferred to make the processing space as narrow as possible (see, e.g., Japanese Patent Application Publication No. 2009-224775). Further, in order to efficiently perform the supplying of the processing gases into the processing space and the purging, and in order to improve the substitution of the gases in the processing space so as to ensure control over film thickness, it has been contemplated that it is necessary to make the pressure in the processing space as low as possible and to increase the flow rate of the purge gas.
However, a film forming apparatus typically has a trap for trapping reaction products flowing in an exhaust line, and the trap limits the substitution rate of gases when the pressure in a processing space is low, so that it is difficult to sufficiently increase the gas substitution efficiency.